Impact of wildfires on the spatial pattern of Larrea tridentata in the Sonoran and Mojave deserts, southwestern USA

Background/Question/Methods

Larrea tridentata (creosote bush) is the dominant perennial shrub throughout most of the arid regions of the southwestern US. Associated with Larrea are “fertility islands” with high amounts of nutrients located under shrub canopies that decrease in amount away from the shrubs. Native annuals tend to grow predominantly under shrub canopies. However, some exotic species, which have invaded the SW American deserts, tend to grow in the open, producing more litter between shrubs. As a result, wildfires have become more frequent and intense with the increase in fuel load connecting shrubs. Little is known about the real impact of wildfires in the spatial distribution of Larrea shrublands and potential implications for the arid plant community. In this research we evaluated how wildfires may affect the spatial distribution of the Larrea shrubs within the Sonoran and Mojave deserts. In each desert, we studied a region that was subjected to a wildfire in 2005 and a nearby control plot. All shrubs locations were recorded with a GPS unit and the condition was noted (living or dead). We performed a point pattern analysis using the Pair Correlation Function g(r) to assess the spatial distribution of the shrubs.

 Results/Conclusions

In unburned areas (controls) of the Sonoran and Mojave deserts, the distribution of Larrea shrubs was regularly spaced at short distances, up to about 3 m, but was randomly distributed at broader scales. In contrast, living Larrea shrubs in burned areas of the Sonoran desert were clumped (aggregated), with the dead shrubs exhibiting a spatial pattern similar to what was observed in the adjacent unburned area. In the Mojave burned area, we found no change in the spatial pattern for the remaining living shrubs, relative to the unburned area. However, dead Larrea shrubs showed evidence of clustering at ca. 2 m and then between 8 and 10 m. We found that the burning pattern was different for each desert. In the Sonoran, fire seems to burn Larrea shrubs in a random way, but leaves clusters of living bushes, whereas in the Mojave fire burned clusters of shrubs at certain distances, but leaves living shrubs randomly distributed. Changes in the spatial pattern of Larrea shrubs may lead to modifications on spatial distribution of nutrients and density of annual plant species, and therefore a new re-distribution on desert plant communities may occur.